246 Rockwell Automation Publication 1789-UM002K-EN-P - January 2015
Appendix D Windows Considerations
The following diagram shows the timing of task execution between two
SoftLogix controllers in the same virtual chassis. Each controller has periodic
tasks and a continuous task. The example periodic tasks are short and take only
0.25 ms to execute. The example continuous tasks take 0.5 ms to execute.
Anytime two periodic tasks need to execute at the same time and they each have
the same priority, they share CPU cycles as the Windows operating system
constantly switches between.
The beginning of the diagram shows what happens when the controller goes
from Program mode to Run mode, which involves a prescan of all tasks. Then
Run mode begins. The real time starts counting as shown at the bottom of the
diagram.
Controller A
Periodic 1ms
Priority 1
Controller A
Periodic 6ms
Priority 2
Controller A
Continuous
Dwell = 2ms
Controller B
Periodic 3ms
Priority 1
Controller B
Continuous
Dwell = 3ms
Dwell Time
Task WaitingTask Running
Prescan
Two Tasks of Equal Priority
Sharing CPU Cycles
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